1. Field of the Invention
The present invention relates to a method and apparatus for material inspection, and more particularly, to a method and apparatus for detecting defects in transparent or translucent material particles and for separating a batch of material particles into portions of like grade or quality.
2. Background of the Invention
Material inspection is an essential part of any manufacturing process. Manufacturers must ensure that the raw materials used in manufacture meet or exceed standards of quality, such as size, color, and purity. Inferior raw materials degrade the quality of the final product and reduce the manufacturer""s sales and profits. Thus, to maintain strict product specifications and satisfied customers, manufacturers demand that raw materials adhere to a minimum quality or grade.
Sensitive to such concerns, suppliers of raw materials routinely conduct inspections with objectives such as identifying and removing flawed material, assessing the overall quality of a batch of material, and separating a batch of material into portions of like size, color, purity, or grade. In the plastics industry, for example, suppliers inspect raw polymer pellets to identify manufacturing defects and to grade a certain lot of pellets for purity. Based on the grading information, suppliers can price lots according to their quality and can offer to manufacturers grades of pellets that meet minimum manufacturing requirements. In this manner, manufacturers do not have to pay for unnecessary purity and, in turn, can maintain competitive product pricing.
When inspecting material, suppliers and manufacturers look for a variety of defects, depending upon the type of material. In food products, for example, defects include foreign matter, uncooked portions, unprocessed or clumped portions, and contaminants from pests such as insects or rodents. In plastic pellets, defects generally include foreign matter, charred raw material, contaminants from unmelted base constituents of the polymer material (often referred to as gels), incorrectly sized or colored pellets, broken pellets, and pellets that are stuck to each other. In addition, manufacturers sometimes measure the amount of fines (small chips or thread-like pieces that can break away from the pellets during manufacturing and transportation).
Traditionally, material inspection has been a slow, labor-intensive process limited to testing small samples instead of all material that is incorporated into the final product. Thus, theoretically, a sample might not be representative of the defects present in the rest of the material. Although the following discussion of the traditional methods of inspection is in the context of the plastics industry, the methods and their associated drawbacks apply equally to other material inspections, e.g., food processing. In the plastics industry, the current methods for inspecting raw plastic material include: 1) visual inspection of pellets by a person; 2) inspection of polymer ribbons formed from pellets; 3) inspection of molten polymer; and 4) automated inspection of the pellets. It is important to note that these methods are typically suitable for base or raw materials that are transparent or translucent. Generally, however, this requirement is not a problem because coloring is usually added late in the manufacturing process.
Visual inspection of pellet material by a person is the most common method of material inspection. It is generally conducted in a quality control laboratory separate from the manufacturing process. The visual inspection method typically involves spreading a sample of particles on top of a light table (e.g., a glass or Plexiglas(trademark) table with a light source below its top) or other white or light-colored surface, and examining each particle for a defect. If the size of a possible defect is small, the inspectors must strain their eyes to observe the defect or perhaps use a magnifying glass to focus on each particle. Although using the light table or light-colored surface enhances the defects, the process is only as reliable as the eyes and concentration of the human inspector. In addition to human error, using human inspectors increases labor costs and significantly reduces speed at which material is analyzed.
Inspection of polymer ribbons involves melting raw material pellets into a molten form, extruding the molten material into thin, ribbon-like shapes, and inspecting the ribbons for defects. The ribbon shapes are flatter than pellets, which eases handling and presents a larger viewable surface area. This ribbon inspection technique can be incorporated into manual (visual inspection by a person) and automated methods of inspection. Despite the advantages in handling and viewable surface area, the ribbon inspection technique suffers from the added time and expense of melting the raw material pellets. The equipment and manpower needed to accomplish this extra step add significantly to the overall cost of material inspection.
In addition to analyzing ribbons, some inspection techniques analyze the molten polymer itself, in a device known as a flow cell. The flow cell is a chamber with a conduit viewable through a window. U.S. Pat. No. 4,910,403 to Kilham and LeBlon discloses a flow cell typically used for the molten polymer inspection technique. The molten polymer is channeled through the conduit, illuminated, and inspected as it passes under the window. This inspection technique can analyze the molten material either manually or with an automated device. Although this method can identify defective portions of the molten polymer, the method cannot separate those defective portions from the remaining acceptable portions. Thus, the method is suitable for grading the molten polymer or monitoring a manufacturing process for quality control, but not for removing defective portions and improving the quality of the molten polymer. In addition, the flow cell and the equipment necessary to convey the molten polymer introduce additional costs and complexities to the inspection process.
Generally, automated inspection of polymer pellets involves passing the material in front of a device that detects defects using technologies such as photography, x-rays, and digital line scanners. For the plastics industry, the typical automated inspection method, which does not include the burdensome step of melting, takes a picture of a pellet as it passes in front of an illuminated background. FIG. 1 shows an example setup of this technique. A camera 100 takes a picture of a pellet 102 as it passes in front of a light source 104. Typically, light source 104 is an illumination source such as the fiber optic backlight disclosed in U.S. Pat. No. 5,187,765. A beam of light 106 illuminates the center of pellet 102 and reaches camera 100. For purposes of explanation and comparison, this application will refer to this automated inspection as the backlighting method.
Although the backlighting method can speed the inspection process, persons skilled in the art recognize that the method is not as accurate as the visual inspection method described above. Principally, the reduction in accuracy is due to the lighting of the round or almost round pellet. Because of the round surface, a clear polymer pellet exhibits a lensing effect that refracts light around the edges of the pellet, in much the same way that images are distorted around the perimeter of a crystal ball or marble. FIG. 2 shows how the light 200 is refracted or redirected as it passes through pellet 102. The pellet refracts light 200 so that the pellet edges appear to be illuminated only by the dull light around light source 104 instead of by the bright light originating from light source 104. As a result of the light refraction, pellet 102 appears darker at its edges than at its center. FIG. 3 illustrates an example of this lensing effect and the shadows that result along the edge of the pellet image. As evident in FIG. 3, if a defect exists at the edge of the pellet, the dark edges caused by the refraction hide the flaw and the automated detection device misses the defect. This error in inspection can lead to the costly consequences of an improperly graded material and an inferior final product.
Thus, there remains a need for an inspection method that incorporates automated inspection for speed and efficiency, yet does not miss defects due to the effect of light refraction by the pellet. The method should avoid the extra cost and time associated with melting the base material and should quickly assess the material and separate out defective portions. Further, the inspection method should maintain the accuracy standards required by material suppliers and product manufacturers.
The present invention is a method and apparatus for automatically inspecting materials. Although described in the context of plastic manufacturing, the present invention could be used in any process that inspects a transparent or translucent particle for defects, such as with food products or glass products. The present invention could also be used to inspect other types of particles, such as white particles, light colored particles, or opaque particles. If such particles are not transparent or translucent, the present invention will detect defects near or at the surface of the particles.
The primary components of the present invention include a light container, at least one camera, a digital processor, and a reject mechanism. Briefly stated, the present invention passes a particle through the light container, captures an image of the particle with the camera, identifies defects in the image with the digital processor, and separates out defective material by communication between the digital processor and the reject mechanism.
The light container is an illumination device that has two opposing openings through which a particle can pass and one or more other openings or windows through which one or more cameras view the particle. Preferably, the light container is an integrating sphere that collects light from an integral input light beam and acts a source of illumination. A commercial example of the light container is disclosed in U.S. Pat. No. 5,537,203, which describes an integrating sphere, constructed of a highly reflective diffuse material, either in the form of a surface coating or as a solid material. U.S. Pat. No. 5,537,203, is incorporated by reference herein. Upon entering the sphere, light from the input beam undergoes multiple reflections, scatters uniformly around the interior of the sphere, and produces spatially integrated light. Preferably, to avoid glares, the input beam transmits light into the light container in a direction away from the location at which the particle passes in front of the camera.
As a particle is dropped or otherwise conveyed through the light container, at least one camera captures an image of the fully illuminated particle. The light container ensures that all sides of the particle are illuminated such that even when viewing the edge of the particle, a line of refracted light still originates from the illuminated wall of the light container. In other words, because the particle is inside the light container, it is backlighted in all directions. The net result is an image of the particle without the dark edges that plague the methods of the prior art. FIG. 4a illustrates an image captured by the present invention with defects noted at points 400 and 402.
In a preferred embodiment of the present invention, the camera is a line scan camera that scans consecutive one-dimensional lines of the particle as the particle travels through the viewing field of the camera. Each one-dimensional line is stored in the memory buffer of the digital processor and assembled into a two-dimensional array. FIG. 4b is a schematic representation of the recording of one dimensional lines, 41 through 48, and the assembling of lines 41 through 48 into a two dimensional image 49. The two-dimensional array captures the image of the particle, like the image shown in FIG. 4a. Although the camera could be of any type, including traditional video cameras, line scan cameras are preferable because of their speed of operation.
In another preferred embodiment of the present invention, multiple cameras view the particle through the light container from varying angles. Multiple cameras reduce the possibility of missing a defect located on the back side of a particle. Such a possibility would be very rare for transparent particles, but could be more common with translucent or opaque particles, e.g., white or gray plastic, which greatly limit the passage of light. The cameras should be spaced equally around the integrating sphere in such a manner that no cameras are opposite each other (in which case a camera would capture the opposite camera in the image, producing a spot that looks like a defect).
The digital processor is a computer that stores data from the line scans of the camera, assembles the line scan data into two dimensional arrays, and looks for dark portions indicative of a defect. If the digital processor finds a defect in a particle, the digital processor directs the reject mechanism to activate at the precise moment the defective particle reaches the reject mechanism. The digital processor can be an individual component of the present invention or can be integrated with the camera as a single component. In this specification and in the claims, the term xe2x80x9ccomputerxe2x80x9d means a portion of a computer with software, a single computer with software, or one or more computers with software in communication with each other.
The reject mechanism is any device that, in response to a signal, separates a defective particle from the stream of particles entering and exiting the light container. Preferably, the reject mechanism is a pneumatic injector that redirects the path of a rejected falling particle with a short blast of air. Also, preferably, the redirected particles fall through a conduit separate from the non-defective particles, thereby creating separate piles or bins of defective and non-defective material.
In a preferred embodiment of the present invention, the above-described components work in concert in an automatic mode so that particles are delivered to the sphere, analyzed for defects, and separated into matching portions without human participation. Optionally, the automatic inspection is an in-line process completed during manufacturing so that all material used in a product is analyzed, not just a sample batch. In this embodiment, the processing speed of the line scan camera and the digital processor would equal or exceed the manufacturing speed of the assembly or manufacturing line.
The present invention is capable of detecting defects that the prior art backlighting devices have not been able to detect. Defects such as charred material and foreign matter appear as dark sections of the particle image, without being obscured by shadowed edges. In addition, as would be apparent to one skilled in the art, a preferred embodiment of the present invention would detect gels within a pellet, by changing the type of light, e.g., using an ultraviolet input light beam to cause the gel defect to fluoresce, and detecting the fluoresence in the visible spectrum by adding filters to the line scan camera.
Accordingly, an object of the present invention is to provide a method and apparatus for inspecting transparent, translucent, or opaque material.
Another object of the present invention is to provide an in-line material inspection process that analyzes all material used in a manufacturing process and identifies and removes all defective portions.
Another object of the present invention is to provide a method and apparatus that sufficiently illuminates a particle to accentuate defects and to eliminate shading that hides defects.
Another object of the present invention is to provide a method and apparatus that eliminates the possibility of missing a particle defect located on a side of the particle opposite to a camera.
These and other objects of the present invention are described in greater detail in the detailed description of the invention, the appended drawings, and the attached claims.